For a while, everything worked out fine. That means 35-50 FPS in Skyrim ultra graphics. But lately, everything slowed down; 15-25 FPS. It is happening in multiple games, and I don't know what to do. I even got a PC cleanup tool.

open Nvidia Control panel and force Nvidia as main GPU. this should fix the issue :)

^This

Open NVIDIA CP, go to Manage 3D Setting. Set "NVIDIA GPU" as the preferred default one.Also in the Global Settings, set the Power Management to "Prefer Max Performance". Be sure to apply your changes. You can always revert back if needed, or for testing, etc.